FIG. 1 illustrates in cross-section a wind turbine rotor blade 10. The blade 10 has an outer shell 11 that is fabricated from two half shells: a windward shell 11a and a leeward shell 11b. The shells 11a, 11 b are moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell 11 are of sandwich panel construction and comprise a core 12 of lightweight foam (e.g. polyurethane) sandwiched between inner and outer GRP layers or ‘skins’ 13, 14.
The blade 10 comprises a first pair of spar caps 15a, 15b and a second pair of spar caps 16a, 16b. The respective pairs of spar caps 15a, 15b and 16a, 16b are arranged between sandwich panel regions 12 of the outer shell 10. One spar cap 15a, 16a of each pair is integrated with the windward shell 11a and the other spar cap 15b, 16b of each pair is integrated with the leeward shell 11b. The spar caps of the respective pairs are mutually opposed and extend longitudinally along the length of the blade 10.
A first longitudinally-extending shear web 17a bridges the first pair of spar caps 15a, 15b and a second longitudinally-extending shear web 17b bridges the second pair of spar caps 16a, 16b. The shear webs 17a and 17b in combination with the spar caps 15a, 15b, 16a, 16b form a pair of I-beam structures, which transfer loads effectively from the rotating blade 10 to the hub of the wind turbine. The spar caps 15a, 15b, 16a, 16b in particular transfer tensile and compressive bending loads, whilst the shear webs 17a and 17b transfer shear stresses in the blade 10.
Figure illustrates in perspective view a conventional spar cap 15a. The spar cap 15a has a substantially rectangular cross section and is made up of a stack of pre-fabricated reinforcing strips 18. The strips 18 are pultruded strips of carbon-fibre reinforced plastic (CFRP), and are substantially flat and of rectangular cross section. The strips 18 are formed by pultrusion, a continuous process similar to extrusion, in which fibres are pulled through a supply of liquid resin and through dies that shape the strip 18. The resin is then cured, for example by heating in an open chamber, or by employing heated dies that cure the resin as the strip 18 is pultruded. The strips 18 have a high tensile strength, and hence have a high load-bearing capacity.
The number of strips 18 in the stack depends upon the thickness of the strips 18 and the required thickness of the shell 11, but typically the strips 18 each have a thickness of a few millimetres and there may typically be between four and twelve strips in the stack. The strips are of decreasing lengths moving from the lower-most strip 18 to the upper-most strip 18, and the ends 19 of the strips 18 are staggered along the length of the spar cap 15a. Each end 19 is tapered, so as to facilitate stress transfer between strips 18 in the stack.
To integrate the components of the wind turbine blade 10, a resin-infusion (RI) process is used. The structural components, including the stacks of strips 18 that will form the spar caps 15a, 15b, 16a, 16b, are laid up in the mould cavity. The components are sealed, a vacuum is applied to the sealed region, and resin is introduced. The vacuum pressure causes the resin to flow over and around the component layers and into the spaces between the stacked strips 18. To complete the process, the resin-infused layup is cured to harden the resin and bond the various laminate layers together to form the blade 10.
The pultruded reinforcing strips 18 described above tend to have a relatively smooth and flat outer surface 20 as a result of the pultrusion process. When the strips 18 are stacked one on top of another in the mould, there is therefore very little space between adjacent strips 18. This lack of space makes it difficult for air to move out of the stack when the vacuum is applied, and air can become trapped between the strips 18 and form air pockets. When resin is infused between the strips 18, the resin infuses around these air pockets, and the lack of space makes it difficult for the resin to push the air pockets out of the stack. The air pockets therefore remain between the strips which results in resin-free areas in the finished component. This is particularly problematic if a resin-free area forms at or near an end 19 of a strip 18. The resin-free area acts as a crack which can propagate under stress, causing delamination of the strips 18, and, in extreme cases, leading to failure of the blade 10.
It is an object of the invention to mitigate or overcome this problem.